The present invention relates to a system for detecting an output of a controllable semiconductor device which is controlled by a magnetic amplifier.
A magnetic amplifier or phase shifter generally has various excellent characteristics such as simple and strong structure, the ability of adding and subtracting a plurality of control inputs while they are isolated from each other and amplifying a resultant output for conversion to a phase shift signal, and the ability of adjusting a time constant so as to avoid the effect of noise. Accordingly, the magnetic amplifier has been widely used to control a controllable semiconductor device such as a thyristor device for an electric car or the like.
U.S. Pat. No. 3,914,672 entitled "Chopper Control System" discloses a chopper control system using a magnetic phase shifter or amplifier to operate a D.C. motor of an electric car.
However, since the system disclosed in the U.S. Pat. No. 3,914,672 is not provided with a function to sense an output of the chopper, the equipments cannot be protected when an excess current flows by commutation failure of the chopper. Also, because there is no function to sense the output of the chopper, it is difficult to effect a field-weakening control which is required to accelerate the motor speed.
In order to enable protection against commutation failure, means for sensing the commutation failure is necessary. However, the commutation failure sensing means is generally complex in its circuit configuration and has a low reliability. Further, since it does not always operate in a fail-safe fashion, the commutation failure sensing means only is not sufficient as a protection device and an additional backup protection device such as an overcurrent relay or the like is required.
Also, in order to properly effect a field-weakening control of a motor, it is necessary to provide means for sensing the maximum duty factor of the chopper, and a similar problem to the above is generally involved in this case too.